Sensitive hybridization techniques have been developed to selectively study the synthesis of avian tumor virus specific RNA in infected and transformed cells. Using these techniques, several size classes of virus-specific RNA can be isolated from the polyribosomes of transformed chicken cells. Experiments currently in progress are attempting to determine 1) the number of discrete size classes present; 2) the oligonucleotide sequence complexity of the different size classes; and 3) the biological activity of these RNAs in an in vitro protein synthesizing system. Experiments in progress are aimed at the isolation of complementary DNA probes specific for a defined region of the RNA tumor virus genome. Construction of such probes will allow the detection and isolation of viral RNA transcripts sequences specific for a defined region of the genome. These virus RNAs can be utilized in in vitro translation studies.